It took Notre Dame staff 23 minutes after the first fire alarm sounded to discover the blaze that went on to cause widespread destruction to the 850-year-old cultural icon.

Key points:

The first fire alarms went off at 6:20pm (local time), but the fire was only discovered after second alarm went off at 6:43pm

Three large stained-glass rose windows were not destroyed, but they will be assessed by an expert for heat damage

There are fears the heat of the fire may have "chemically compromised" some of the cathedral's masonry

Paris public prosecutor Remy Heitz said the cathedral's fire alarms went off twice on Monday evening, beginning at 6:20pm (local time).

A Paris judicial official, speaking anonymously, said staff checked under the roof after the first alarm and saw nothing — but when the fire was discovered 23 minutes later, after the second alarm, it was already too late to stop the inferno.

France's deputy interior minister Laurent Nunez said saving the cathedral came down to a key timeframe of between 15 and 30 minutes, praising the work of firefighters who contained the blaze.

More than $1 billion has been pledged to the restoration of Notre Dame cathedral, with French President Emmanuel Macron saying he wanted to see the cathedral rebuilt in five years.

Mr Macron said Notre Dame would be "even more beautiful" after the restoration, however experts said the staggering amount of cash flung at rebuilding efforts might not be enough to replace what was lost.

Professor Peter McPhee, a specialist in French history at the University of Melbourne, said he feared "that the sheer heat of that fire may have chemically compromised some of the masonry" in the historic building.